1. Field of the Invention
This invention relates to the field of integrated circuits. More particularly, this invention relates to on-board diagnostic circuits for integrated circuits.
2. Description of the Prior Art
It is known to provide on-board diagnostic circuits for integrated circuits. One example of such known circuits are the serial scan chains controlled by a JTAG-type controller as used by ARM Limited of Cambridge, England in several of its microprocessor designs. A problem with known scan chain type diagnostic circuits is that the serial scanning in and scanning out of data is insufficiently quick to allow real time and/or efficient diagnostic operations. In particular, it may be necessary to slow an integrated circuit below its normal operating speed, or halt it, so that the serial diagnostic data may be scanned in and scanned out of the serial scan chains. Operating an integrated circuit below its normal operating speed has the result that the diagnostic operation performed is not truly representative of real functional operation of the integrated circuit. It may be that certain faults only manifest themselves at the full operating speed and will be invisible at the lower speeds at which the circuit must operate to accommodate the use of diagnostic serial scan chains. Further using the JTAG as communications to the processor for debug creates at least a two-step approach to the access of system resources.